1. Field of the Invention
This invention relates generally to the field of borehole drilling and more particularly to an improved method and apparatus for drilling a curved bore in drilling lateral drain holes in order to increase well production.
2. Description of the Prior Art
One way to achieve enhanced production from depleted wells is to drill short lateral holes known as "drain holes" from a main bore into a desired interval. The main bore and the horizontal drain hole are connected by a section of curved bore which builds at some predetermined angle such as 2.degree. or 3.degree. per foot. The apparatus for performing this task usually comprises several structures interconnected in descending order of the lower end of a drill pipe, including one or more lengths of flexible shaft, a knuckle joint, a reamer, and a drill bit. The initial direction of the curved bore is determined by a deflecting tool or whipstock. Since the diameter of the knuckle joint is less than that of the reamer and drill bit, the joint will be pushed toward the bottom edges of the deflecting tool. This places the common bit-reamer axis at an angle to the deflecting tool causing the bit, as soon as it clears the whipstock, to bite more sharply into the top bore surface, thus causing the bore to curve.
In order to be able to drill a drainhole in this manner so that the laterally or horizontally extending portion there of will be at the desired depth, it is essential to be able to predict accurately what the "build" angle will be. Failure to do so may lead to total failure of the drainhole to produce. This in turn requires correct determination of the relationship between the radius of curvature of the borehole and the geometry of the bottomhole drilling structure.
In U.S. Pat. No. 3,398,804 relating to a method of drilling a curved bore, utilizing the drill assembly elements described above, it is assumed that the borehole proceeds on a smooth curve which maintains the reamer in a position perpendicular to the radius of the curved path of the borehole. It is believed that this concept is in error and leads to incorrect calculations of borehole curvature. An equation for calculating borehole curvature and build angle must correctly identify the point of tangency between the direction of the bottomhole assembly and the borehole.
It is therefore an object of this invention to provide an equation for correctly correlating the borehole curvature and the bottomhole assembly dimensions.
A further problem in drilling drainholes arises when drilling into intervals of significantly varying hardness. For example, drilling operations in one instance revealed that a drainhole drilling assembly built to about 35.degree. and then fell off to vertical before building again. This, of course, completely nullified any calculations concerning drainhole depth. This problem is generally attributed to the fact that the lateral forces generated by moving from one layer of the formation to another of different hardness tend to destabilize the bottomhole assembly and cause the reamer and bit to drill laterally instead of following concentrically along the correct path.
It is therefore a further object of this invention to provide a method and apparatus for minimizing the destabilizing effect on a drainhole drilling apparatus of passing between drilling intervals of varying hardness.
Other objects and advantages of this invention will become apparent from a consideration of the detailed description to follow taken in conjunction with the drawings and appended claims.